Jump to content


Photo

PEC - This can't be right! NEQ6

  • Please log in to reply
11 replies to this topic

#1 nobbygon

nobbygon

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 20 Mar 2009
  • Loc: Australia

Posted 12 December 2012 - 07:41 PM

Hi,

I've recently pier mounted my NEQ6 and last night finished my polar alignment so that the star wouldn't move over 10 mins. I've never tried PEC before so decided to give it a go. My results are attached. I captured the data on a DSI II Pro and an Orion short tube 80 (FL 400mm)using PHD. Shot on a star on the meridian near the equator.

I can’t believe that my PE is only +9 -12 arcsecs. I was expecting more like +- 30. What could I have done to get incorrect data that would show a PE less than what it really is? Could it be that the scope that I shot the data through has a focal length that is too short?

I wanted to see how my guiding was going and to my surprise I've never seen it as good as this before. OSC index of 0.19 and RMS of 0.12. I've never seen numbers that good before. Maybe it’s down to a really accurate polar alignment. Any thought here?

Cheers for any help,
Angus.

Attached Files



#2 nobbygon

nobbygon

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 20 Mar 2009
  • Loc: Australia

Posted 12 December 2012 - 07:42 PM

And the guiding graph

Attached Files



#3 orlyandico

orlyandico

    Fly Me to the Moon

  • *****
  • Posts: 5486
  • Joined: 10 Aug 2009
  • Loc: Singapore

Posted 12 December 2012 - 09:35 PM

+10 / -12 is a very good figure. It is possible to get an NEQ6 with that level of precision.

The only reasons I can see for mis-reporting of this figure is (1) not factoring in the declination when importing the PHD log into PECPrep; (2) incorrect pixel size when importing the PHD log; for example the Meade DSI has 9.6 x 7.5 um pixels. If the wrong axis gets chosen when calculating, your PE would be proportionately larger or smaller; and (3) incorrect focal length of the guide scope.

However, given your data (400mm FL and DSI Pro II) that's an image scale of 4.27" per pixel. RMS of 0.12 means you are guiding at (4.27 * 0.12) = 0.51" ! that is an amazing figure, comparable to a Mach1.

The best I can do on my CGEM is 0.18 RMS @ 9.71" / pixel which is 1.75" RMS. This is about 3X your figure. But then again my CGEM has +/- 20" of PE.

#4 korborh

korborh

    Viking 1

  • *****
  • Posts: 802
  • Joined: 29 Jan 2011
  • Loc: Arizona

Posted 12 December 2012 - 09:42 PM

Is that star saturated?

#5 andysea

andysea

    Apollo

  • *****
  • Posts: 1497
  • Joined: 03 Sep 2010
  • Loc: Seattle, WA

Posted 12 December 2012 - 10:59 PM

Yeah that's about right. I get 0.3"~0.5" RMS with my mounts.
I think it means that the PE is easily guided out right? That NEQ6 is a keeper!

Andy

#6 nobbygon

nobbygon

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 20 Mar 2009
  • Loc: Australia

Posted 12 December 2012 - 11:12 PM

Yeah I'm prety sure the star is saturated. If not it's pretty close. Does that have anyhting to do with how PHD guides?

#7 korborh

korborh

    Viking 1

  • *****
  • Posts: 802
  • Joined: 29 Jan 2011
  • Loc: Arizona

Posted 13 December 2012 - 01:22 AM

If star is saturated, the centroid calculated by PHD would be incorrect and hence show its not moving. All those saturated pixels and large size of the star can mask the movement of the centroid as information is lost.

#8 gezak22

gezak22

    Viking 1

  • *****
  • Posts: 897
  • Joined: 15 Aug 2004
  • Loc: On far side of moon. Send help.

Posted 13 December 2012 - 01:42 AM

Yeah I'm prety sure the star is saturated. If not it's pretty close. Does that have anyhting to do with how PHD guides?


Under 'Tools' there is an option 'Auto select Star' or something like it. It automatically selects the star with the highest SNR in the guide image.

#9 andysea

andysea

    Apollo

  • *****
  • Posts: 1497
  • Joined: 03 Sep 2010
  • Loc: Seattle, WA

Posted 13 December 2012 - 07:32 AM

Yeah I'm prety sure the star is saturated. If not it's pretty close. Does that have anyhting to do with how PHD guides?


Under 'Tools' there is an option 'Auto select Star' or something like it. It automatically selects the star with the highest SNR in the guide image.

Does that really work? I'm always afraid that PHD will pick a hot pixel by mistake. I have to say I never tried that option tho.

Andy

#10 EFT

EFT

    Vendor - Deep Space Products

  • *****
  • Vendors
  • Posts: 2545
  • Joined: 07 May 2007
  • Loc: Phoenix, AZ

Posted 13 December 2012 - 10:30 AM

A mount with +/-30 would actually be a pretty bad example. Your results are on the better side of average which I would consider to be somewhere in the +/-15 to 20 range. Pier mounting and good alignment have not hurt you either.

#11 gezak22

gezak22

    Viking 1

  • *****
  • Posts: 897
  • Joined: 15 Aug 2004
  • Loc: On far side of moon. Send help.

Posted 13 December 2012 - 11:10 AM

Yeah I'm prety sure the star is saturated. If not it's pretty close. Does that have anyhting to do with how PHD guides?


Under 'Tools' there is an option 'Auto select Star' or something like it. It automatically selects the star with the highest SNR in the guide image.

Does that really work? I'm always afraid that PHD will pick a hot pixel by mistake. I have to say I never tried that option tho.

Andy


You bet it works. The reason why it will NEVER select a hot pixel is because PHD knows what hot pixels look like (isolated pixels) while stars have gaussian pixel distributions. PHD is pretty clever.

#12 andysea

andysea

    Apollo

  • *****
  • Posts: 1497
  • Joined: 03 Sep 2010
  • Loc: Seattle, WA

Posted 13 December 2012 - 05:23 PM

Ah! I guess I should have read all the documentation :o
I will let PHD pick a star the next time I'm imaging!!






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics